Many agents used in the treatment of human malignancy are known to induce chromosome aberrations, resulting in cell death of the tumor cells and clinical response. At the same time, chromosome aberrations are also produced in normal tissue, and this can result in treatment-associated morbidity (e.g. myelosuppression and mucositis) and increased risk for the development of second malignancy. The long-term goal of this research is to improve the therapeutic index of the treatment of malignant disease, i.e. maximize damage to tumor tissue and minimize damage to normal tissue. To achieve this goal, it is important to better understand the molecular mechanisms involved in chromosome aberration formation and repair after treatment with therapeutic agents. This proposal focuses on two general issues. First, the relationship between chromatin structure and sensitivity to therapeutic agents is addressed by examining Ataxia telangiectasia cells for alterations in chromatin structure that impact initial levels of DNA and chromosome damage and the cell's ability to repair this damage. In addition, the chromatin of cells is altered by Topoisomerase II antagonists and its effects on chromosome damage, repair and survival of irradiated normal and tumor cells is determined. The working hypothesis behind this approach is that the manner in which DNA is folded up into chromatin can affect the efficiency of conversion of DNA double strand breaks into chromosome breaks as well as their repairability. DNA and chromosome damage and repair is determined by the DNA filter elution and premature chromosome condensation techniques, and chromatin organization is probed at the nucleoid level. The second focus of the proposal is a determination of the consequences of inhibition of chromosome repair with metabolic inhibitors such as F-AraA, with regard to its long term consequences on residual levels of chromosome damage as well as cell survival. Chromosome studies here will be enhanced with fluorescence in situ hybridization using chromosome-specific probes ('chromosome painting'). These latter-studies will yield significant new information regarding the molecular mechanisms of chromosome damage and repair as well as provide a potential approach to increasing the therapeutic ratio in the treatment of malignant disease. Overall, the proposed studies will provide basic insights into chromosome aberration mechanisms as well as clinically relevant information for therapeutic strategies and the long term effect of therapy on normal tissues.